The present invention relates to a process for preparing methacrylic acid based on the hydrolysis of methacrylic esters.
The prior art discloses a multitude of processes for preparing methacrylic acid.
A customary procedure consists in the controlled oxidation of hydrocarbon gases, for example propylene or butylene. A disadvantage of these processes is the yields obtained thereby, which are relatively low when viewed overall.
In addition, methacrylic acid can be obtained by the reaction of methacrylamide with water. This process is described more particularly in U.S. Pat. No. 7,253,307. According to this publication, the reaction of methacrylamide with water can be effected in a stirred tank reactor or a tubular reactor. The reaction is preferably performed at a pressure of 3.65 to 7.70 bar and a temperature in the range from 50 to 210° C.
The processes described in U.S. Pat. No. 7,253,307 for preparation of methacrylic acid already lead to good yields with a high purity. However, methacrylic acid is an important product in the chemical industry, which serves as a starting material for many important products. Therefore, a maximum yield and a particularly high purity with low production costs are essential for the economic success of such a preparation process. Even relatively small improvements with regard to the yields, the service lives of the plants or similar process features lead to a significant advance with regard to the amounts of waste and the preparation costs.
α-Hydroxyisobutyric acid can likewise serve as a starting material for preparation of methacrylic acid. Such a process is described, for example, in U.S. Pat. No. 3,487,101, where the preparation of various methacrylic acid derivatives, especially methacrylic acid and methacrylic esters, proceeding from 2-hydroxyisobutyric acid (HIBA) in the liquid phase, is characterized in that the conversion of HIBA to methacrylic acid is performed in the presence of a dissolved basic catalyst at high temperatures between 180-320° C. in the presence of high-boiling esters (e.g. dimethyl phthalate) and internal anhydrides (e.g. phthalic anhydride). According to the patent, at HIBA conversions of >90% MA selectivities around 98% are achieved. No information is given as to the long-term stability of the liquid catalyst solution, more particularly the exhaustion of the anhydride used.
DE-A 1 191 367 relates to the preparation of methacrylic acid (MA) proceeding from HIBA in the liquid phase, characterized in that the conversion of HIBA to methacrylic acid is performed in the presence of polymerization inhibitors (for example copper powder) and in the presence of a catalyst mixture consisting of metal halides and alkali metal halides at high temperatures between 180-220° C. According to the patent, at HIBA conversions of >90%, MA selectivities of >99% are achieved. The best results are achieved with catalyst mixtures of zinc bromide and lithium bromide. It is common knowledge that the use of halide-containing catalysts at high temperatures places severe demands on the materials to be used, and these problems with regard to the entrained halogenated by-products present in the distillate also occur in downstream plant parts.
EP 0 487 853 describes the preparation of methacrylic acid proceeding from acetone cyanohydrin (ACH), characterized in that, in the first step, ACH is reacted with water at moderate temperatures in the presence of a heterogeneous hydrolysis catalyst and, in the second step, α-hydroxyisobutyramide is reacted with methyl formate or methanol/carbon monoxide to form formamide and methyl hydroxyisobutyrate (MHIB), and in the third step MHIB is hydrolysed in the presence of a heterogeneous ion exchanger with water to give HIBA, and, in the fourth step, HIBA is dehydrated, by allowing it to react in the liquid phase at high temperatures in the presence of a soluble alkali metal salt. Methacrylic acid preparation from HIBA is described at high conversions around 99% with more or less quantitative selectivities. The multitude of reaction steps necessary and the necessity of intermediate isolation of individual intermediates, more particularly also the performance of individual process steps at elevated pressure, make the process complicated and hence ultimately uneconomic. In addition, formamide is inevitably obtained, this compound in many cases being viewed as an unwanted by-product which has to be disposed of expensively.
DE-A 1 768 253 describes a process for preparing methacrylic acid by dehydration of HIBA, characterized in that HIBA is converted in the liquid phase at a temperature of at least 160° C. in the presence of a dehydration catalyst which consists of a metal salt of HIBA. Particularly suitable in this case are the alkali metal and alkaline earth metal salts of HIBA, which are prepared in situ in an HIBA melt by conversion of suitable metal salts. According to the patent, MA yields up to 95% from HIBA are described, the feed to the continuous procedure consisting from HIBA and approx. 1.5% by weight of HIBA alkali metal salt.
RU 89631 relates to a process for preparing methacrylic acid proceeding from HIBA by elimination of water in the liquid phase, characterized in that the reaction is performed in the absence of a catalyst with an aqueous solution of HIBA (up to 62% by weight of HIBA in water) under pressure at high temperatures of 200° C.-240° C.
There have additionally been detailed studies of the use of propene as a base raw material, obtaining methacrylic acid in moderate yields via the stages of hydrocarbonylation to isobutyric acid and dehydrogenating oxidation.
It is known that propanal or propionic acid, which are obtainable in industrial processes proceeding from ethylene and C-1 units such as carbon monoxide, can be used as a base raw material. In these processes, in an aldolizing reaction with formaldehyde, the β-hydroxycarbonyl compound formed in situ is dehydrated to the corresponding α,β-unsaturated compound. An overview of the standard processes for preparing methacrylic acid and the esters thereof can be found in the literature, for example Weissermel, Arpe “Industrielle organische Chemie” [“Industrial Organic Chemistry”], VCH, Weinheim 1994, 4th edition, p. 305 ff. or Kirk Othmer “Encyclopaedia of Chemical Technology”, 3rd edition, Vol. 15, page 357.
It is therefore an object of the invention to provide a novel process for preparing methacrylic acid, which does not have the disadvantages mentioned, more particularly produces smaller amounts of waste acid, has a lower energy consumption, and enables higher yields and a lower water content in the end product.